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Refinement of the Crystal Structure of the Aragonite Phase of CaCO3
1Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234.
Abstract:
Aragonite (CaCO3) crystallizes in the unit cell a = 4.9598(5) Å, b = 7.9641(9) Å, and c = 5.7379(6) Å at 25 °C with four formula weights in space-group Pmcn. The structure has been refined to R = 0.024, R = 0.040 using 765 x-ray reflections from a single crystal. The Ca ion is coordinated to nine oxygens with Ca … O distances in the range 2.414(2) Å to 2.653(1) Å. The two unique C-O distances in the CO3 group are 1.288(2) Å (on the mirror plane) and 1.283(1) Å. The two unique O-C-O angles are 119.6(2)° (across the mirror plane) and 120.13(8)°. The distances and angles in the CO3 group are not significantly different at the 95 percent confidence level.
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